Kollektorstrom

Kollektorstrom, in electronics, refers to the current that flows through the collector terminal of a bipolar junction transistor (BJT). It is usually denoted Ic and is a key parameter in describing a transistor’s operation. By convention, Ic is taken as positive for NPN transistors in forward-active operation, while for PNP transistors the direction is opposite.

In the forward-active region, the collector current Ic is largely controlled by the base current Ib. The current gain, or beta, relates the two through Ic approximately equal to beta times Ib (Ic ≈ β Ib). The emitter current Ie is the sum of base and collector currents (Ie ≈ Ib + Ic), and the common-collector current gain α is defined as α = β/(β + 1), with Ic ≈ α Ie. These relationships form the basis for using a BJT as an amplifier or switch.

Ic also depends on the collector-emitter voltage Vce. The Early effect describes a small increase of Ic with increasing Vce due to the widening of the base-collector depletion region, which affects the transistor’s effective gain. In saturation, both the base-emitter and base-collector junctions are forward biased, and Ic is limited by external circuit conditions rather than Ib alone; the transistor no longer follows the simple Ic ≈ β Ib relation.

Practically, Ic determines the output current in amplifier stages and the on-state current in switching circuits. For MOSFETs, the analogous current is the drain current Id, since BJTs use collector current.